1. Field of the Invention
The present invention relates generally to the imaging of materials, and more specifically to the imaging of materials by nuclear magnetic resonance.
2. Description of the Prior Art
NMR imaging is used primarily for medical applications and is known in that context as Magnetic Resonance Imaging (MRI). In most MRI applications, (i) the NMR image is from the hydrogen nuclei from rather mobile water species in the body; (ii) the body is placed within a large, homogeneous magnet and within a set of switchable, or time-varying, magnetic field gradient coils and; (iii) the signal is excited (and sometimes detected) by rf coils which surround the body. There have already been some approaches to the general problem of imaging materials and to the more specific problems of "one-sided" imaging (in which the sample is outside both the magnet and rf coil).
NMR imaging of large planar structures requires that the material be accessed from one side. To date no one-sided (planar) NMR imaging schemes have been demonstrated, although one-sided NMR spectroscopy (in which the entire signal from a remote region is examined, but without any spatial discrimination) has been used for several applications. There are a number of possible modes for planar NMR imaging of materials. These may be roughly broken down into two categories: one requires a homogeneous magnetic field and the other requires an inhomogeneous magnetic field. The present inventors have already published a method for imaging materials with NMR which is amenable to planar imaging within a homogeneous magnetic field. This mode of imaging is referred to as rf selected planar imaging.
That earlier method for imaging materials in a homogeneous magnetic field employs one-sided rf excitation and detection, in which the sample is within a homogeneous magnet but outside only the rf coil. The method relies on inhomogeneous rf excitation for spatial localization of the signal. All spins within the coil volume are excited but only spins from a small region are left observable after excitation. Thus spins in a volume which is of roughly planar cross-section and parallel to the plane of the rf surface coil are observed. This region is called the volume of interest (VOI). The dimensions of the VOI are determined by the rf excitation scheme and by the coil geometry. If the VOI has a sufficiently small depth, it may be viewed as essentially planar. Likewise, if the VOI also has a sufficiently small diameter, the VOI is essentially one-dimensional. Nevertheless, because even that essentially one-dimensional region maintains a measurable depth and diameter, and because the dimensions of the region of interest can be varied depending upon the requirements for precision, even an essentially one-dimensional region of interest is called a VOI.
The drawback to the rf selected technique is that all planes parallel to the rf surface coil are excited simultaneously while only one is detected: the signal from regions outside the VOI is lost and the spin system must return to equilibrium before the next plane can be detected. This return to equilibrium is determined by the spin-lattice relaxation time T.sub.1 and for materials can take on the order of several seconds to minutes.
NMR imaging in large static magnetic field gradients has also been reported. In the experiments reported to date the imaging has been accomplished inside a homogeneous magnet but at the edge of the magnet coil such that the magnitude of the field is dropping rapidly. The sample was also contained inside a solenoidal rf coil. Detection was accomplished by keeping the NMR probe tuned to a single frequency and moving the sample through the magnetic field gradient.